Semiconductor devices have recently been developed to have a microstructure, and thus they show a marked tendency to be easily damaged or burned with an applied stress, such as Joule heat or the like. This damage of the semiconductor device occurs when a voltage for a breakdown-voltage measurement is applied to a device under test (DUT). That is, when a breakdown-voltage measurement is carried out for a diode or other element, the DUT is temporarily left in a break-down state many times, so that a current flowing through the DUT is regarded as a pulsed current. The DUT suffers from not only a micro-stress due to the pulsed current, but also from a macro-thermal stress due to heat which is caused by the pulsed current and the breakdown voltage.
Both manual and automatic measuring operations have been conventionally adopted for a breakdown voltage of a semiconductor device, and a manner of depressing the above stress as much as possible has been utilized. A curve tracer, for example, is used in the manual measuring operation. As shown in FIG. 1, a DUT is supplied with a half-wave voltage V.sub.HS that has a peak value higher than a breakdown voltage V.sub.BD of the DUT in the neighborhood of a peak portion of the sine wave, as indicated by an oblique line portion of FIG. 1. The breakdown voltage at a predetermined current value is visually measured by an operator. Representing a period of V.sub.HS and a duration time of the breakdown state by T and t.sub.BD respectively, the ratio of the breakdown duration to the measuring time is equal to t.sub.BD /T. For example, in the case where a visual observing time is 3 seconds and t.sub.BD /T is 0.1, the breakdown state is continued over 300 ms. Therefore, the stress described above would not be removed for an extended period, and thus the DUT would be damaged or burned.
On the other hand, a ramp voltage generator, for example, may be used in the automatic measuring operation. In this operation, the rise-up of a ramp voltage is set to start at a predetermined time t.sub.O, as shown in FIG. 2, and a predetermined voltage is applied to the DUT. When a response voltage V reaches a prescribed value, the DUT suffers from a breakdown,.and a breakdown current rapidly flows through the DUT. At this time, a current limiter interposed between the ramp voltage generator and the DUT is actuated to limit the current flowing through the DUT to a predetermined value, whereby the response voltage V of the DUT is controlled to remain constant. Then, after a predetermined amount of time has elapsed from the time t.sub.O, the response-voltage V at a time t.sub.11 is measured to obtain the breakdown voltage V.sub.BD at the predetermined current. A ramp rate (RR) of the ramp voltage is determined in consideration of (1) the slew rate of a measuring system, (2) an electrostatic capacity of the DUT and (3) a set value I.sub.LIM of the current limiter.
The breakdown voltage is divergent, even for similar kinds of DUTS, and frequently varies, e.g., from 10 V to 100 V, which could be problematic. For example, in comparison of response voltages V.sub.A and V.sub.B of DUTs A and B of FIG. 3B, the ramp rates of the respective DUTs are equal to each other, but the response voltage V.sub.B of the DUT having a lower breakdown voltage suffers from breakdown at an earlier time than the response voltage V.sub.A. Therefore, a time interval T.sub.B from the breakdown time of V.sub.B to a measuring time t.sub.M is longer than a corresponding time interval t.sub.A. Further, in comparison of response voltages V.sub.B and V.sub.C of DUTs B and C, the response voltage V.sub.C has a higher breakdown voltage than the response voltage V.sub.B, but the interval (or measuring time) T.sub.C of V.sub.C is longer than the measuring time T.sub.B of V.sub.B because V.sub.C has a higher ramp rate than V.sub.B.
Therefore, measurements under the same conditions cannot be performed for the DUTs A, B and C, and thus there occurs a problem that a uniform measurement procedure can not be used for all DUTS. In addition, in a case where the breakdown continues for a long time (e.g., the case of DUTs B and C whose response voltages are shown in FIG. 3B), the stress becomes excessive; that is, the DUT is damaged or the breakdown voltage varies due to the stress itself. The breakdown voltage also varies due to heat which is caused by the stress. Therefore, there occurs a problem in that the breakdown voltage under a prescribed condition can not be accurately measured. Moreover, there is a further problem in that an expensive ramp voltage generator capable of altering a ramp rate must be used.
Accordingly, an object of the present invention is to overcome the above disadvantages, and in particular to provide methods and apparatus, for measuring a breakdown voltage, by which thermal stress is largely reduced and highly accurate measurements are obtained. A further object of the present invention is to provide means for measuring a breakdown voltage under uniform measuring conditions, so that automatic measurements may be obtained with high reproducibility.